A lawsuit filed in a California appeals court on Thursday alleges the ballots of as many as 45,000 voters weren’t counted last November because of the state’s flawed rules for verifying the signatures of those who vote by mail.

The lawsuit was filed by the ACLU of Northern California on behalf of a Sonoma County voter who said his ballot wasn’t counted after his signature on the ballot envelope was deemed to not match the one that elections officials had on file.